This invention relates to a process and an apparatus for purifying and recovering waste gas treating chemicals.
Waste gas treating chemicals such as alkanolamines, are used in aqueous solution which flows countercurrent to natural gas for removing acid gases such as hydrogen sulfide and carbon dioxide, and/or water vapor from the gas. Thereafter, the gas treating chemical flows countercurrent through steam in a regenerator or stripper in order to remove absorbed material from the treating chemicals. Over a period of time, certain contaminants accumulate in the solution. When the contaminant level is such that the solution is no longer effective for removing acid gases, or when the removal of such acid gases and regeneration becomes too expensive or too corrosive, the gas treating chemical must be replaced.
The contaminants commonly found in the gas treating chemicals include products of the thermal degradation of the gas treating chemicals, heat stable salts, asphaltenes, light hydrocarbons, suspended solids or combinations thereof. The degradation products are high boiling nitrogen and oxygen compounds, and the heat stable salts include sodium thiosulphate, sodium thiocyanate and sodium sulphide. In order to reclaim the gas treating chemical, it is necessary to remove all such impurities.
However, when attempting to reclaim gas treating chemicals, it must be understood that all gas treating chemicals are susceptible to decomposition at elevated temperatures, and any attempt to remove impurities at a temperature of 400.degree. F. or higher will result in thermal decomposition of the gas treating chemical. The temperature of 400.degree. F. has been established as the temperature above which decomposition becomes measurable. Moreover, most gas treating chemicals are corrosive, which condition is exacerbated by high temperatures and long residence times in the reclaiming apparatus.
An example of a waste gas treating chemical reclaiming process is found in U.S. Pat. No. 2,914,469, which issued to H. M. Anderson et al on Nov. 24, 1959. The Anderson et al method uses potassium hydroxide which is added to a contaminated diethanolamine solution to form an eutectic mixture of molten salt. The process is carried out at atmospheric pressure and at a temperature of from 450.degree. to 460.degree. F.